Machining / interactive calculator

Tap Drill Size Calculator

Estimate a starting drill diameter for 60-degree metric and Unified cut threads. Adjust thread engagement and switch units without losing your values.

01 / Thread inputs

Live calculation

Nominal thread diameter

Major diameter of the internal thread.

Thread pitch

Axial distance between adjacent thread crests.

Target thread engagement

A planning value; higher engagement increases tapping torque.

Tap type

Form taps require the tap manufacturer's recommended hole chart.

Formula & assumptions

How the tap drill estimate is calculated

Metric 60-degree thread

Drill = D - (0.01299 x engagement % x pitch)

D and pitch are entered in millimetres. At 75% engagement this is close to the familiar D minus pitch shop rule.

Unified inch thread

Drill = D - (0.01299 x engagement %) / TPI

D is in inches. This estimates geometry for a 60-degree cut thread; select a real available drill at or above the calculated value as the process requires.

Production application disclaimerConfirm the drill against the tap manufacturer's chart, thread class, material, coating, hole depth, and required engagement. Do not use this cut-tap estimate for form or roll taps.

From a theoretical diameter to an available drill

For an M10 × 1.5 cut thread at an illustrative 75% theoretical engagement, the calculation is 10 − (0.01299 × 75 × 1.5) = 8.538625 mm. That is a geometric result, not an instruction to buy an 8.538625 mm drill or a finished-hole tolerance.

Trial hole diameter	Engagement from the same geometric model
8.50 mm	(10 − 8.50)/(0.01299 × 1.5) = 76.98%
8.60 mm	(10 − 8.60)/(0.01299 × 1.5) = 71.85%

The two stocked choices do not produce the same theoretical engagement. Neither is automatically acceptable for the specified thread. Check the tap manufacturer’s permitted hole range, thread class and material application before choosing. Then verify the actual drilled hole: a drill’s nominal marking does not account for runout, wear or the process’s size variation.

An inch-thread cross-check

For a 1/4-20 cut thread at 75%, the equation gives 0.250 − (0.01299 × 75/20) = 0.2012875 in, or 5.1127025 mm. A #7 drill is nominally 0.2010 in, so the familiar chart choice is close but not numerically identical.

Browse the 83-size tap drill chart or read how engagement is calculated. Form/roll taps need a different pre-hole specification; do not reuse this cut-thread example. Source: OSG tap drill and thread charts.